Charge air cooling is in widespread use in supercharged piston engines, in particular large diesel engines with turbocharging. The cooling of the precompressed combustion air increases the density of the intake working medium and therefore the charge in the cylinders. In the case of coolers which are based on heat exchange with ambient air, cooling water or other cooling media, the dissipation of enthalpy from the cycle process has proven to be disadvantageous. It is known from U.S. Pat. No. 4,440,116 or GB 2 259 326 for water or a water-alcohol mixture to be injected into the working medium upstream or downstream, respectively, of the compressor of a turbocharger. The evaporation extracts heat from the intake air and the overall volume of the working medium decreases. The heat which has been extracted remains present in the water vapor which is formed and in this way is not initially lost to the cycle process. Evaporation cooling of this nature can also be used for other purposes, for example described in U.S. Pat. No. 4,664,091. The injection of water for cooling purposes and secondly as an inert medium in the intake section or directly in the combustion chamber of engines with internal combustion allows the formation of nitrogen oxides to be reduced. Therefore, the tendency toward the feared knocking combustion can be reduced in piston engines which are operated with spark ignition, i.e. for example in the case of operation with carburetor fuels or fuel gases. Particularly in the case of very hot and dry ambient conditions, the cooling of the intake air by introduction and evaporation of water may be advantageous even for engines which are not supercharged or other engines with internal combustion, in order to increase the filling of the combustion chambers and to reduce the compression work required. In this context, it may also be of interest for so much water to be introduced into the intake air that drops are still present even during the compression phase. The evaporation of these drops during the compression ensures intensive internal cooling during the compression process and therefore leads to a further reduction in the compression work. The water vapor which is formed, on the other hand, performs useful work during expansion. This overall effect of increasing output was observed as early as the 1940s and 1950s, when preliminary coolers or intercoolers in closed gas turbine cycles were leaky, so that cooling water entered the compressor.
One problem of the methods which have been discussed for conditioning the working medium is the need to continuously provide water which has to satisfy high purity requirements and which is expelled with the exhaust gas in the case of an engine with internal combustion, so that it is lost. For example, in the case of diesel engines or gas turbine sets which are used to drive vehicles, it is then always necessary for a sufficiently large water tank to be transported to cover the demand for water over a certain driving distance or operating time.
Furthermore, in gas turbine plants, there are known methods of recuperating waste heat from the exhaust gas by injecting steam into the working medium. For example, U.S. Pat. No. 5,689,948 has disclosed a power plant in which the waste heat from a gas turbine is used to generate steam. The steam is introduced into the pressurized working medium of the gas turbine set at a suitable location and is expanded in the gas turbine so as to release useful power. With techniques of this nature, it is possible to achieve very good characteristic values with regard to efficiency and output for relatively little outlay. A drawback is the high consumption of water, since the water vapor which is generated is expelled to atmosphere together with the exhaust gas. Especially in arid areas, the quantities of water required cannot be provided in the long term or at least can only be provided at very high economic cost. The abovementioned methods for cooling the intake working medium before or during the compression can also advantageously be used in conjunction with gas turbine sets.
Furthermore, it is also known to introduce water or steam into the combustion chamber of a gas turbine set in order to reduce the levels of nitrogen oxide emissions.
It is also known to use steam to cool the highly thermally loaded components of a gas turbine set and to allow this steam to flow out into the working medium after cooling has taken place.
In each of the methods mentioned, however, a water mass flow is fed to the working medium of an engine with internal combustion—irrespective of whether this be the intake air, the partially compressed or compressed combustion air or the hot gas—and according to the prior art this water mass flow is expelled to atmosphere together with the exhaust gas, in other words the expanded working medium. Consequently, high-quality water in quantities of quite easily several tonnes per hour have to be provided for methods of this nature. However, this is often very difficult or impossible to realize for political, economic or even moral ethics and social reasons, and consequently it is often impossible to use technically viable and desirable solutions.
Therefore, in U.S. Pat. No. 5,843,214 it is proposed for water which has been introduced into the working medium of an engine with internal combustion—for example in an STIG gas turbine—to be condensed in the exhaust gas after the working medium has been expanded and then returned to the working medium. However, autonomous operation is of course only possible if the moisture contained in the exhaust gas is sufficient for an exhaust-gas condenser to produce a water mass flow which at least corresponds to the water or steam mass flow required to be introduced into the working medium. Therefore, it is necessary for the working medium to be enriched with moisture before autonomous operation can be achieved.